1. Field of the Invention
The present invention disclosed in this specification relates to an organometallic complex. In particular, the invention relates to an organometallic complex that can provide light emission from a triplet excited state. In addition, the invention relates to a light-emitting element and a light-emitting device each using the substance.
2. Description of the Related Art
In recent years, there have been the active research and product development of light-emitting elements in each of which an organic or inorganic compound having a light-emitting property is used as a light-emitting material. Especially light-emitting elements called EL (electroluminescence) elements have characteristics such as feasibility of being thinner and more lightweight and responsive to input signals, because the basic structure of such elements is a simple structure in which a layer containing a light-emitting material (a light-emitting layer) is just provided between a pair of electrodes (an anode and a cathode).
The light emission mechanism of an organic EL element is as follows: voltage application between a pair of electrodes causes electrons injected from the cathode and holes injected from the anode which serve as carriers to recombine in the light emission center of a light-emitting layer, a light-emitting substance is brought into an excited state, and energy is released when the molecular excitons return to the ground state; thus light is emitted. An organic EL element is also referred to as a carrier-injection element because of this mechanism.
A singlet excited state (S*) and a triplet excited state (T*) are known as types of the above excited state, and light emission can be obtained through either of the excited states. Note that, in a light-emitting element, the statistical generation ratio of the singlet excited state (S*) to the triplet excited state (T*) is considered to be 1:3.
It can be estimated in terms of the above generation ratio that, when the number of the injected carriers is 100%, about 25% of the light (photons) emitted by a light-emitting element is light emission from the singlet excited state (S*) and about 75% is light emission from the triplet excited state (T*).
Note that in this specification, light emission from the singlet excited state (S*) is referred to as “fluorescence” and a compound that emits fluorescence is referred to as a “fluorescent compound”. Further, in this specification, light emission from the triplet excited state (T*) is referred to as “phosphorescence” and a compound that emits phosphorescence is referred to as a “phosphorescent compound”.
Thus, use of a phosphorescent compound as well as a fluorescent compound for a light-emitting element enables the ratio of generated photons to injected carriers (hereinafter, referred to as internal quantum efficiency) to be increased to 75 to 100%. In other words, it is possible to realize three to four times as high emission efficiency as that of an element using only a fluorescent compound. For such a reason, light-emitting devices including light-emitting elements using phosphorescent compounds have been under active development in recent years so that highly-efficient light-emitting elements can be realized (e.g., see Non-Patent Document 1). As phosphorescent compounds, organometallic complexes that have iridium or the like as a central metal have particularly attracted attention because of their high phosphorescence quantum yield.
Further, a light-emitting element using a phosphorescent compound is disclosed which uses a light-emitting layer containing an organic low molecular hole-transport substance and an organic low molecular electron-transport substance as host substances and the phosphorescent compound as a dopant and has improved emission efficiency (see Patent Document 1).
The mainstream research have focused on application of light-emitting devices including such light-emitting elements having high emission efficiency to image display devices typified by organic EL displays.
At the same time, due to recent attention to the energy problems, power consumption is becoming a major factor controlling the trends in consumer purchases and application of light-emitting elements to lighting devices is also being actively examined.
In application of light-emitting elements to lighting devices, it is preferable to use color with high luminance (the luminance means the degree of brightness perceived by humans and is expressed as the proportion of the degree of the brightness of a perceivable wavelength on the assumption that the wavelength of light which is the most strongly perceived by human eyes (555 nm) is 1). One of such color with high luminance is yellow (Y).
Note that yellow (Y) has advantageous effects on image display devices as well, since addition of yellow to the three primary colors (red (R), green (G), and blue (B)) for image display devices creates features such as higher luminance and more brilliant display of dark yellow or bright green.